1. Field of the Invention:
The present invention relates to a tracking servo apparatus for performing tracking servo with respect to a recording medium having recording tracks.
2. Description of the Prior Art:
In order to perform recording/reproducing of data along a recording track in a non-contact state with respect to a recording medium such as an optical disk, tracking servo must be performed to track a beam spot along the recording track.
In a conventional tracking servo apparatus, a tracking error signal obtained when a beam spot crosses a track has a waveform as a signal V.sub.P of FIG. 1A. Assuming that a displacement of the beam spot from the track center is x and a track pitch is p, the signal V.sub.P is given by the following equation: ##EQU1## In this case, tracking servo is performed so that the signal V.sub.P becomes 0.
A track jump for moving a beam spot to an adjacent track or another track separated by several tracks to perform data recording/reproducing is performed as follows in the conventional tracking servo apparatus.
First the beam spot is moved close to a target track in an open-loop state wherein tracking servo is turned off. Then, tracking servo is turned on to track the beam spot to the target track.
However, the signal V.sub.P is a sine wave and hence cannot be uniquely determined with respect to the displacement x, as shown in FIG. 1A. The signal V.sub.P is uniquely determined with respect to the displacement x only in the hatched regions in FIG. 1A. That is, the beam spot can stably follow the track only when it is located in the hatched regions in FIG. 1A.
If the displacement x is large and the beam spot is located outside the hatched regions, tracking servo is destabilized or oscillated. Such a state tends to occur especially when a relative moving speed of the beam spot in a track pitch direction is large as in a track jump.
For example, this is a case wherein an error is produced in a moving distance of the beam spot when a track jump is performed with tracking servo off, and tracking servo is turned on outside the hatched regions in FIG. 1A.
In this case, since tracking servo cannot be easily stabilized after a track jump, it takes a long period of time to track the beam spot to the target track. In addition, the beam spot may be undesirably tracked to a track other than the target track. At this time, a track jump must be performed again.